Boat thruster systems which utilize pump, pipe and valve arrangements for drawing water in from the sea and selectively discharging water through port and starboard openings in the hull of the boat are well known. Typical systems such as described in U.S. Pat. Nos. 4,056,073 and 4,214,544 employ valves located within such pipes to selectively direct water to the port and/or starboard hull openings. Such systems typically include closed loop valve control systems utilizing feedback circuitry for precisely positioning the valves to achieve various proportions of maximum thrust.
Although open loop valve control systems would be simpler, less expensive, and easier to maintain than closed loop systems, it has been recognized that substantial system damage can occur in the event of a failure which permits both outlet pipe valves to be closed at the same time.